Almost all major epidemiological and intervention studies have incriminated elevated levels of blood cholesterol and LDL-C as significant risk factors for coronary morbidity and mortality, while HDL-C has a protective role. Attempts to reduce the risk for coronary heart disease have emphasized the amount of total fat, saturated (SFA) and polyunsaturated (PFA) fatty acids and cholesterol in the diet. An effect of dietary monounsaturated fatty acids (MFA) - hitherto believed to have no influence on blood lipids - has been recently proposed. However, it has not been definitely demonstrated that this effect of dietary MFA was due to enrichment with MFA specifically, or the reduction in dietary SFA, increase of the P:S ratio or other secondary changes in the diet composition. We propose to investigate the specific effect of dietary MFA on lipoprotein structure and function in a free-living population of young health Israeli males (aged 18-24 yr), living on a college (yeshiva) campus. Five different experiments will be performed to compare the effects of a diet with a high MFA content and one with a low MFA content, as follows: 1) High MFA - low PFA vs. Low MFA - high PFA; 2) High MFA vs. Low MFA - high starch; 3) High MFA vs. Low MFA - high SFA 4) High MFA vs. Low MFA; 5) High MFA vs. Low MFA - low fat. The amounts of cholesterol, sugar and fiber will be similar in all diets. Each subject will consume two of the diets for a 3-month period in a cross-over design study. The specific objectives are to study the influence of dietary MFA on blood lipids, lipoprotein classes and subfractions, their structure and composition, their uptake by specific receptors, their degradation by cultured fibroblasts, and their reverse transport in plasma. Changes in the fatty acid content of cells and lipoproteins will be followed and their susceptibility to peroxide formation measured. In addition, the effect of dietary MFA on non-lipid parameters of atherogenesis such as platelet function will also be evaluated.